linux-kernel - Re: [tip:core/urgent] WARN_ON_SMP(): Add comment to explain ({0;})

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Message-ID: <AANLkTinh6nraZND1mQJ4ofPbABOtJNEnGYYEkqs92RQM@mail.gmail.com>
Date:	Mon, 28 Mar 2011 16:56:59 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	torvalds@...ux-foundation.org, rostedt@...dmis.org,
	srostedt@...hat.com, tglx@...utronix.de, adobriyan@...il.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/urgent] WARN_ON_SMP(): Add comment to explain ({0;})

On Mon, Mar 28, 2011 at 4:51 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 03/28/2011 07:45 AM, tip-bot for Steven Rostedt wrote:
>>
>> WARN_ON_SMP(): Add comment to explain ({0;})
>>
>> The define to use ({0;}) for the !CONFIG_SMP case of WARN_ON_SMP()
>> can be confusing. As the WARN_ON_SMP() needs to be a nop when
>> CONFIG_SMP is not set, including all its parameters must not be
>> evaluated, and that it must work as both a stand alone statement
>> and inside an if condition, we define it to a funky ({0;}).
>>
>> A simple "0" will not work as it causes gcc to give the warning that
>> the statement has no effect.
>>
>> As this strange definition has raised a few eyebrows from some
>> major kernel developers, it is wise to document why we create such
>> a work of art.
>>
>
> What the heck is wrong with the idiomatic and non-gcc-extension-using:
>
>        ((void)0)
>
> ?

AFAIK you cannot use it within an if-statement.

>        -hpa
>
> --
> H. Peter Anvin, Intel Open Source Technology Center
> I work for Intel.  I don't speak on their behalf.
>
> --
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-- 
Thanks,
//richard
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